Leadless integrated circuit package having standoff contacts and die attach pad

ABSTRACT

A leadless integrated circuit (IC) package comprising an IC chip mounted on a die attach pad and a plurality of electrical contacts electrically connected to the IC chip. The IC chip, the electrical contacts, and the die attach pad are all covered with a molding material, with portions of the electrical contacts and die attach pad protruding from a bottom surface of the molding material.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 12/775,711 (issued as U.S. Pat. No. 8,736,037),filed May 7, 2010 entitled, “Leadless Integrated Circuit Package HavingStandoff Contacts and Die Attach Pad.” U.S. patent application Ser. No.12/775,711 is a divisional application of U.S. patent application Ser.No. 12/400,391 (issued as U.S. Pat. No. 7,858,443), filed Mar. 9, 2009entitled, “Leadless Integrated Circuit Package Having Standoff Contactsand Die Attach Pad.”

BACKGROUND

1. Technical Field

This invention relates generally to integrated circuit (IC) packagingtechnology, and more particularly, to leadless IC packages and relatedmethods of manufacture.

2. Description of Related Art

IC packaging is one of the final stages involved in the fabrication ofIC devices. During IC packaging, one or more IC chips are mounted on apackage substrate, connected to electrical contacts, and then coatedwith a molding material comprising an electrical insulator such as epoxyor silicone molding compound. The resulting structure—commonly known asan “IC package”—is then connected to other electrical components, forexample, on a printed circuit board (PCB).

In most IC packages, the IC chip is completely covered by the moldingmaterial, while the electrical contacts are at least partially exposedso that they can be connected to other electrical components. In otherwords, the electrical contacts are designed to form electricalconnections between the IC chip inside the molding material, andelectrical components outside the molding material. One of the mostcommon designs for these electrical contacts is one in which they form“leads” extending out the sides of the molding material. The leadstypically are bent downward to form connections with electricalcomponents on a PCB.

Unfortunately, the presence of external leads tends to significantlyincrease the size of IC packages. For instance, it tends to increase thelength and width across the IC packages due to the horizontal extensionof the leads. This increased size can be disadvantageous in systemswhere PCB space is limited. In addition, because the external leads aretypically arranged along the sides of the IC packages, the pin count ofthe IC packages is limited by the linear distance around the ICpackages. Additionally, these leads require an additional inspectionstep for straightness, coplanarity and other required mechanicaldimensions (and rework or scrap if they fail the specification).Finally, the leads (starting from the bonding fingers down to the tip ofthe external portions) add to the total electrical signal length (bondwires+leads), which affect the electrical performance of the IC chip.

Recognizing these and other problems with conventional IC packages,researchers have developed IC packages in which the external leads arereplaced by electrical contacts that are covered on top by the moldingmaterial, but exposed on the bottom of the IC package so they can beconnected to electrical components located beneath the IC package. TheseIC packages—referred to as “leadless” IC packages—tend to occupy lessspace compared with conventional IC packages due to the absence of theexternal leads. In addition, these IC packages eliminate the need forbending the leads to form connections.

Some examples of conventional leadless IC packages are disclosed inrelated and commonly assigned U.S. Pat. Nos. 6,498,099 and 7,049,177,the respective disclosures of which are hereby incorporated byreference. Among other things, these patents describe and illustratevarious design variations for leadless IC packages and varioustechniques for manufacturing and using the leadless IC packages.

SUMMARY

The disclosed embodiments include leadless IC packages in which theelectrical contacts extend below a molding layer, forming “standoff”contacts. These IC packages may also include die attach pads formed fromthe same leadframe strip as the electrical contacts and extending belowthe surface of the molding layer.

These die attach pads may be exposed through the molding layer to form adirect thermal path from the IC chip down to a copper layer on the PCB.The copper layer may act as a heat sink to dissipate heat generated bythe IC chip during normal operation. The exposed die attach pads mayvary in size, depending on the size of the IC chip and if ground bondingdown to an inside portion of the pad is required. In some embodiments,the exposed die attach pad may be much smaller than the IC chip in orderto make room for vias or routing on the PCB underneath the package. Inother embodiments, it may be desirable to not have any exposed dieattach pad at all, since the presence of exposed metal, in closeproximity to circuit traces on a PCB underneath the package may resultin interference or undesirable electrical coupling. In yet otherembodiments, the exposed die attach pad may have special configurations,such as an array of segmented portions, in order to facilitate surfacemounting to the PCB or to provide special electrical connectivity withinthe die attach pad. In the case of segmented portions, channels createdbetween such segments may also allow out gassing of soldering fluxduring surface mounting.

In one embodiment, a leadless IC package comprises a die attach pad, anIC chip mounted on the die attach pad, a plurality of electricalcontacts electrically connected to the IC chip, and a molding layerformed over the IC chip, the die attach pad, the electrical contacts,and materials used for connection of the IC chip to the electricalcontacts, such as bonding wires. The molding layer has a top partcovering top portions of the die attach pad and the electrical contacts,and a bottom part through which bottom portions of the die attach padand electrical contacts are exposed. At least some of the bottom portionof the die attach pad extends out of the bottom part of the moldinglayer by a smaller distance compared with the bottom portions of one ormore of the electrical contacts.

In another embodiment, a leadless IC package comprises an adhesivelayer, an IC chip mounted on the adhesive layer, a plurality ofelectrical contacts electrically connected to the IC chip, and a moldinglayer formed over the IC chip, the adhesive layer, and the electricalcontacts. The molding layer has a top part covering top portions of theelectrical contacts, and a bottom part through which a bottom surface ofthe epoxy layer and bottom portions of the electrical contacts areexposed. The electrical contacts extend out of the bottom part of themolding layer and the adhesive layer is formed in a footprint of themolding layer created by a die attach pad formed from a common metallayer with the electrical contacts.

In yet another embodiment, a method of manufacturing a leadless ICpackage comprises removing portions of a leadframe strip to formrecesses defining areas for a die attach pad and a plurality ofelectrical contacts. An IC chip is mounted in the die attach pad areaand then electrical connections are formed between the electricalcontact areas and the IC chip. The IC chip, the die attach pad area, andthe electrical contact areas, and the electrical connections are coveredwith a molding layer. An etch resist layer is formed over the electricalcontact areas on a bottom surface of the leadframe strip, and a bottomsurface of the leadframe strip is selectively etched using the etchresist layer as an etching mask, thereby forming the electrical contactsand the die attach pad as separate components. The selective etching ofthe bottom surface of the leadframe strip removes at least a portion ofthe die attach pad such that a bottom portion of the die attach padextends away from a bottom surface of the molding layer by a smallerdistance compared with bottom portions of one or more of the electricalcontacts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a leadless IC package that serves as a reference forexplaining other leadless IC packages presented in this writtendescription.

FIGS. 2A and 2B illustrate a leadless IC package having a partiallyetched die attach pad that is at the same level as a bottom surface of amolding layer.

FIGS. 3A and 3B illustrate a leadless IC package from which a die attachpad has been completely removed and covered with a protective and/orinsulating layer which is shown as a black substance.

FIGS. 4A and 4B illustrate a leadless IC package from which a die attachpad has been completely removed as in FIG. 3 and a recess is formed in afootprint of the die attach pad.

FIGS. 5A and 5B illustrate a leadless IC package from which peripheralportions of a die attach pad have been etched away and the bottom of amiddle portion of the die attach pad is at the same level as the bottomof external contacts.

FIGS. 6A and 6B illustrate a leadless IC package from which peripheralportions of a die attach pad have been etched by a first amount and amiddle portion of the die attach pad has been etched by a second amount.

FIGS. 7A-7B, 8A-8B, and 9A-9B illustrate various leadless IC packages inwhich a die attach pad has been patterned to form a plurality ofdiscrete protrusions.

FIGS. 10A-10C illustrate various leadless IC packages having die attachpads and electrical contacts formed in various alternative shapes andconfigurations.

FIGS. 11A-11B, 12A-12B, 13A-13B, and 14A-14B illustrate various leadlessIC packages having solder material formed on the bottom of correspondingdie attach pads and electrical contacts.

FIGS. 15A-15E illustrate a method of manufacturing a leadless IC packagein accordance with one embodiment.

FIGS. 16A-16E illustrate a method of manufacturing a leadless IC packagein accordance with another embodiment.

DETAILED DESCRIPTION

Selected embodiments are described below with reference to theaccompanying drawings. These embodiments are provided as teachingexamples and should not be construed to limit the scope of the claims.In the drawings, like reference numbers denote like features. Where likefeatures are included in more than one of the illustrated embodiments,these features may be described only once in order to avoid redundancy.

In general, the embodiments relate to leadless IC packages havingstandoff contacts and/or die attach pads arranged in variousconfigurations. These different configurations can be used to achieveany of several different purposes, such as avoiding electrical and/orphysical interference between the die attach pads and circuit traces ona PCB underneath the package, improving heat-transfer characteristics ofthe die attach pads, facilitating surface mount to the PCB, providingspecial electrical connectivity within the die attach pad and/orelectrical contacts and addressing space-based IC packaging constraintssuch as pin count, to name but a few.

In the description that follows, a number of embodiments are presentedin specific configurations, such as multiple-row quad fine-pitch no-leadpackage (QFN) configurations where the standoff contacts are arranged inmultiple staggered rows on four sides of an IC chip. The disclosedconfigurations, however, are intended for illustration only and not forany limiting purpose. For example, more than one IC chip may be attachedto the die attach pad in a side-by-side configuration, known asmultichip module (MCM). The die attach pad itself may be one piece orsegmented, in cases where multiple chips that may not share the samebackplane. Alternatively, IC chips may be stacked one on top of theother in stacked die configurations. Further, in cases where the packageneeds to be as small as possible, the package may not have a die attachpad at all. For instance, the IC chip may be attached directly andpartially on top of the electrical contacts using anelectrically-insulative adhesive (known as chip on lead). The IC chipmay also be attached directly to the electrical contacts using flipchiptechniques, wherein the bond pads have solder bumps which may bereflowed to upper surfaces of the electrical contacts.

Some embodiments are also described with specific components such ascertain types of IC chips including semiconductor processor die.However, these and other components may be substituted with other parts,modified, or supplemented with additional components. For example,passive components, such as chip resistors and capacitors, may beattached to the electrical contacts along with the IC chips (system inpackage). Moreover, specific materials in the described embodiments,such as certain types of metals, could be substituted with similarmaterials.

For convenience of explanation, this description includes a number oforientation-specific terms, such as “top,” “bottom,” “over,” “on,” andso forth. These terms should not be construed to limit the orientationof the described articles, but are intended merely to reflect therelative positions of various components or portions thereof. Forinstance, a “bottom surface” may be interpreted to mean a surfaceopposite a “top surface,” regardless of the orientation of the articlehaving the bottom and top surfaces.

FIG. 1 illustrates an example of a leadless IC package 100 in accordancewith one embodiment. The embodiment shown in FIG. 1 is used as areference for explaining a number of additional embodiments below. Thus,the reference numbers shown in FIG. 1 are also used in connection withseveral other figures.

Referring to FIG. 1, an IC package 100 comprises an IC chip 105, a dieattach pad 110, electrical contacts 125, wire bonds 130, and a moldinglayer 120. IC chip 105 is mounted on die attach pad 110 by an adhesivelayer 115. Adhesive layer 115 may comprise, for instance, a polymericmaterial such as epoxy, silicone, polyimide and thermoplastic materials(both in paste or film form), or a soft solder material such as gold-tinor various combinations of tin and lead alloys. IC chip 105 maycomprise, for instance, a processor die or a memory chip cut from asemiconductor wafer.

IC chip 105 includes bonding pads 140 that act as input/output (I/O)terminals and are connected to electrical contacts 125 via wire bonds130. Both die attach pad 110 and electrical contacts 125 may be covered,on a top surface, by a bondable metal layer 135 which may be plated.Bondable metal layer 135 may comprise a metals such as a stack-up ofnickel (Ni), palladium (Pd) and gold (Au), or a stack-up of nickel (Ni)and gold (Au) or silver (Ag). Bottom surfaces of die attach pad 110 andelectrical contacts 125 may be plated by the same metal layer as the topsurface, or covered with other metal finishes such as silver (Ag), gold(Au), nickel (Ni) and gold (Au), electrolytic or immersion tin (Sn),tin/lead (Sn/Pb), tin alloy or other solder finishes, or hot dip or barecopper (Cu) with a coating of a organic solderability preservative(OSP). Layer 135 may serve any of several functions, including, forinstance, enhancing wire bondability of a top layer, protecting theplated surfaces against oxidation, improving solderability, andimproving electrical conductivity.

Molding layer 120 covers IC chip 105, wire bonds 130, die attach pad110, and electrical contacts 125, but leaves portions of die attach pad110 and electrical contacts 125 exposed on a bottom surface of ICpackage 100. The exposed portions of die attach pad 110 and electricalcontacts 125 protrude from molding layer 120 to form a “standoff” dieattach pad and “standoff” contacts, respectively. In other words, theterm “standoff” indicates that electrical contacts 125 and die attachpad 110 protrude, or “stand off”, from the bottom surface of moldinglayer 120 by a measurable distance, e.g., 0.0005 to 0.020 inches.

The exposed part of die attach pad 110 can be used as a solder surfacefor soldering IC package 100 to a PCB or another substrate. It can alsoserve as a thermal dissipation surface or heat sink for drawing heat offof IC chip 105 during operation. The exposed portions of electricalcontacts 125, on the other hand, can create electrical connectionsbetween IC chip 105 and electrical components outside IC package 100.

Die attach pad 110 and electrical contacts 125 are typically formed froma single strip of metal such as copper. This strip—denoted a “leadframe”strip—can be used as a platform for simultaneously manufacturing severalIC packages. For instance, several IC packages can be simultaneouslymanufactured by etching patterns for multiple die attach pads andcorresponding electrical contacts on a single leadframe strip, placingIC chips on the die attach pads, wire bonding the chips to theelectrical contacts, coating the entire structure with a molding layer,and then separating the resulting structure into individual IC packages,e.g., by saw singulation or die punching. Because die attach pad 110 andelectrical contacts 125 are typically formed from the same leadframestrip, they also typically (though not always) have the same initialthickness.

IC package 100, once mounted on a PCB or other substrate, may be locatedin close proximity to other electrical components. For instance, it maybe placed directly over components such as routing wires in order tomaximize the use of space on the PCB. If IC package 100 is close enoughto other components, the exposed portions of die attach pad 110 andelectrical contacts 125 may create electrical and/or physicalinterference with the other components. As an example, the exposedportion of die attach pad 110 may cause electrical shorts or capacitiveinterference in the routing wires due to its location and/or shape.Accordingly, various embodiments described below include features thatmay reduce the incidence of such interference. In addition, die attachpad 110 may also be advantageously patterned to achieve improved circuitrouting capabilities on a PCB underneath the IC package. Moreover, thepatterning of die attach pad 110 may provide channels through whichgases from a solder paste may escape during a heating process when theIC package is attached to the PCB or other substrate. When trapped,these gases have been known to cause large voids and/or lift the ICpackage, causing an unreliable solder attachment. A number of thedescribed embodiments include features that may contribute to theseimproved capabilities.

FIG. 2A illustrates a leadless IC package 200 in accordance with anotherembodiment. IC package 200 has a number of similarities with IC package100, as indicated by the common reference numbers. However, in ICpackage 200, the protruding portion of die attach pad 110 has beenremoved by chemical etching or mechanical grinding to produce arelatively thinner die attach pad 210 that is substantially flush with abottom surface of molding layer 220. In addition, a top portion of dieattach pad 210 is plated with metal layer 135 to form a ground ring 245for ground bond application on die attach pad 210.

IC package 200 can be connected to a PCB or another substrate bysoldering one or more of electrical contacts 125 to corresponding PCBlands. Alternatively, die attach pad 210 may remain detached from (notsoldered to) the PCB or substrate. Because die attach pad 210 is thinnerthan electrical contacts 125, it can maintain a distance from electricalcomponents or circuit traces on the PCB or substrate located below ICpackage 200 to avoid creating electrical or physical interference.

FIG. 2B illustrates the bottom of IC package 200 when it is formed in astaggered multi-row QFN configuration with electrical contacts 125 onfour sides. As seen in FIG. 2B, die attach pad 210 covers a relativelylarge surface area at the bottom of IC package 200, allowing circuittraces or other components on the PCB underneath the area defined by ICchip 105 to be present without electrical or physical interference withthe exposed die attach pad 210. Contacts 125 are formed in two staggeredrows around die attach pad 210 to create high terminal density whileallowing the wire bonds 130 that connect electrical contacts 125 with ICchip 105 to be adequately spaced apart from each other in accordancewith the staggered configuration.

FIG. 3A illustrates a leadless IC package 300 in accordance with anotherembodiment. IC package 300 is also similar to IC package 100, with thefollowing differences.

In IC package 300, IC chip 105 is mounted directly to a metal strip,such as copper, on a central portion of the strip circumscribed byelectrical contacts 125. IC chip 105 is attached using an adhesive layerof black-colored epoxy or another polymer material, such as silicone,polyimide or thermoplastic (either in paste or film form) prior to theformation of molding layer 120. Before IC chip 105 is mounted on thecentral portion of the copper strip circumscribed by electrical contacts125, a top portion of the copper strip is pre-etched so that IC chip 105can be mounted in a plane located below a top surface of electricalcontacts 125. Molding layer 120 is subsequently formed over IC chip 105,wire bonds 130, and electrical contacts 125. After molding layer 120 isformed, the remaining portion of the die attach pad is completelyremoved by chemical etching or mechanical grinding. Thereafter, thesurface below IC chip 105 is covered with an epoxy layer 310. In thecurrent example, epoxy layer 310 is the same color as molding layer 120(e.g., black) so that IC chip 105 is not visible from the bottom of ICpackage 300, as illustrated by the large blank spot at the middle ofFIG. 3B.

FIG. 4A illustrates a leadless IC package 400, which is a variation ofIC package 300. In IC package 400, IC chip 105 and epoxy layer 310 arerecessed away from the bottom surface of molding layer 120 by 5 to 30%of the molded thickness—e.g., 1 to 3 mils (28 to 76 microns). The depthof the recess can be controlled, for instance, by varying the amount ofpre-etching of the die attach pad. Less pre-etching may result in adeeper recess and more pre-etching may result in a more shallow recess.

The recess is created by removing the remaining die attach pad usingchemical etching or mechanical grinding. After the die attach pad iscompletely removed, the bottom surface of IC chip 105 is covered withepoxy layer 310, which may be the same color as molding layer 120. Theresulting structure has a footprint 410 on its bottom surface, as seenin FIG. 4B.

FIG. 5A illustrates a leadless IC package 500 in accordance with anotherembodiment. IC package 500 is similar to IC package 100, except that inIC package 500, a peripheral portion 510 of the exposed die attach pad110 has been thinned by chemical etching or mechanical grinding. As aresult, the outer edges of die attach pad 110 are substantially flushwith the bottom surface of molding layer 120. The peripheral portion 510may also be flush with or even recessed from the bottom of molding layer120. FIG. 5B shows a bottom surface of IC package 500, including theperipheral portion 510 that has been thinned on die attach pad 110. Notethat this thinned down portion 510 increases the physical separationbetween contacts 125 and exposed die attach pad 110 and allows for thepresence of circuit traces or vias on the PCB directly below that areaof IC package 500 and reduces the chance of shorting between the dieattach area and the 1^(st) row of electrical contacts during solderingof the IC package to the PCB or substrate.

FIG. 6A illustrates a leadless IC package 600 in accordance with anotherembodiment. IC package 600 is similar to IC package 100, except aperipheral portion 610 and a central portion 615 of die attach pad 110have both been thinned by chemical etching or mechanical grinding suchthat peripheral portion 610 of die attach pad 110 is flush with thebottom surface of molding layer 120, and central portion 615 remainsslightly thicker than peripheral portion 610. In other words, centralportion 615 has been etched or grinded to a first depth and peripheralportion 610 has been etched or grinded to a second depth greater thanthe first depth. FIG. 6B shows a bottom surface of IC package 600,including the portions 610 and 615 that have been thinned on die attachpad 110.

FIGS. 7A and 7B illustrate a leadless IC package 700 in accordance withanother embodiment. IC package 700 is similar to IC package 100, exceptthat die attach pad 110 has been etched in specific areas to form apattern illustrated in FIG. 7A and FIG. 7B. The patterning of die attachpad 110 forms discrete protrusions 715 at regular intervals on thebottom of IC package 700. Protrusions 715 may be referred to as dieattach pad electrical contact members (DAP ECMs), although they are notnecessarily used as electrical contacts. However, these DAP ECMs 715have the same exposed bottom size and shape as contacts 125, which allowthe same amount of solder paste to be dispensed, thereby facilitatingsurface mount to the PCB. Another benefit of these discrete protrusionsis that they provide channels through which the out gassing of thesoldering flux can escape during the surface mount operation.

FIGS. 8A and 8B illustrate a leadless IC package 800 similar to leadlessIC package 700. IC package 800, however, includes protrusions 815 thatare narrower and shorter compared with protrusions 715 in IC package700. As a result, the bottom surfaces of protrusions 815 lie in a planethat is nearer to molding layer 120 than are the bottom surfaces ofelectrical contacts 125.

FIGS. 9A and 9B illustrate a leadless IC package 900 similar to leadlessIC package 700. IC package 900, however, includes discrete protrusions915 in die attach pad 110 that are wider and shorter compared withprotrusions 715 in IC package 700. As an alternative to the wider andshorter protrusions 915, IC package 700 could also be modified toinclude protrusions that are longer and narrower, longer and shorter, orwider and longer. In other words, the discrete protrusions 915 can haveany number of size and shape variations, depending on the electricalcircuit requirements, although the protrusions typically do not extendlonger than electrical contacts 125. FIG. 9B shows the bottom of ICpackage 900 including the wider and shorter protrusions 915.

FIGS. 10A, 10B, and 10C illustrate various alternative configurations1000 of electrical contacts 125 and DAP ECMs. The alternativeconfigurations of electrical contacts 125 can be achieved, for instance,by modifying the etching pattern of the leadframe strip when electricalcontacts 125 are initially defined. The alternative configurations ofthe DAP ECMs, on the other hand, can be achieved by patterning dieattach pad 110 in different ways either before or after molding layer120 has been formed.

FIG. 10A shows a configuration 1005 of die attach pad 110 withnon-uniformly shaped protrusions that form rectangles or squares ofdifferent sizes when viewed from a bottom surface. FIG. 10A also shows aconfiguration 1010 of electrical contacts 125 in which some of thecontacts form rectangles or squares of different sizes when viewed froma bottom surface.

FIG. 10B shows a configuration 1020 of electrical contacts 125 in whicheach contact has a round profile when viewed from a bottom surface. Inthe example of FIG. 10B, die attach pad 110 has not been patterned.

FIG. 10C shows a configuration 1025 of die attach pad 110 withnon-uniformly shaped protrusions that form circles and ovals or ellipseswhen viewed from a bottom surface. FIG. 10C also shows a configuration1030 of electrical contacts 125 in which some of the contacts form ovalsor circles when viewed from a bottom surface.

Die attach pad 110 and electrical contacts 125 could be formed with anumber of alternative shapes and patterns other than those shown inFIGS. 10A-10C. For instance, they could be formed with combinations ofround and rectilinear shapes, larger and smaller shapes, and so on. Inaddition, they could be formed into custom shapes and patterns dependingon the desired electrical characteristics and/or connectionspecifications of a particular IC package.

FIGS. 11A and 11B illustrate a leadless IC package 1100 similar toleadless IC package 100. In IC package 1100, however, the electricalcontacts 125 and die attach pad 110 have a solder material 1105 attachedto their respective contact surfaces. The shaded portions in FIG. 11Bindicate the solder material attached to die attach pad 110 and each ofelectrical contacts 125. The addition of solder material facilitatesreplacement of bad IC packages during a rework process of a populatedPCB. Reworking a PCB may involve removal of the defective IC package,site dressing or cleaning of the PCB area from which the defective ICpackage was removed, fluxing of the site, placement of the replacementIC package 1100 and solder reflow. Without solder material 1105, a userwould have to manually dispense the solder material onto the replacementIC package using a manual stencil, since selective screen printing ofthe solder material onto the PCB is no longer possible due to thepresence of other components on the said PCB.

In some embodiments, solder material 1105 comprises a tin and leadsolder alloy. In other embodiments, solder material comprises a non-leadsolder alloy such as Tin-Silver, Tin-Silver-Copper,Tin-Silver-Copper-Nickel or a combination of other high-melt soldersknown to those skilled in the art.

FIGS. 12A and 12B illustrate a leadless IC package 1200 similar toleadless IC package 1100, except that in IC package 1200 electricalcontacts 125 and die attach pad 110 have been reduced in size by etchingor grinding before the attachment of solder material 1105. Thisembodiment combines the advantages discussed in relation to IC packages500 and 1100.

FIGS. 13A and 13B illustrate a leadless IC package 1300 similar toleadless IC package 1200. In IC package 1300, however, electricalcontacts 125 and die attach pad 110 have been removed by etching orgrinding until they are flush with the bottom surface of moldingmaterial 120 prior to the attachment of solder material 1105.

FIGS. 14A and 14B illustrate a leadless IC package 1400 similar toleadless IC package 700, except that in IC package 1400, the bottom ofdie attach pad 110 and electrical contacts 125 are not covered withplating 135 prior to the application of a solder material 1405, whichmay be applied in a paste form and reflowed to form a plurality ofsolder bumps on the discrete protrusions of die attach pad 110 andelectrical contacts 125. The attachment of solder material 1405 can bemade using any of several well-known application techniques, such asscreen printing or pattern-write needle dispensing.

As an alternative to attaching solder material 1405 in the paste form,it could be attached to electrical contacts 125 and the protrusions ofdie attach pad 110 in the form of preformed solder balls. The solderballs may be attached using a solder paste or flux material and a reflowprocess. The flux or solder paste material ensures proper solder wettingof die attach pad 110 and electrical contacts 125 during the reflowprocess.

Any of the embodiments illustrated in FIGS. 11-14 could be modified sothat solder is only applied to a subset of the bottom contact surfacesof electrical contacts 125 and die attach pad 110. For instance, in anumber of embodiments, die attach pad 110 may not be soldered at all.Similarly, in some embodiments, not all of electrical contacts 125 willbe soldered down.

FIGS. 15A-15E illustrate a method of manufacturing a leadless IC packagesuch as IC package 200 illustrated in FIG. 2. The method may be used tosimultaneously manufacture several leadless IC packages with a singleleadframe strip. However, for simplicity, only one IC package will beillustrated and described. In the description that follows, examplemethod steps will be denoted by parentheses (xxxx) to distinguish themfrom example IC package components.

Referring to FIG. 15A, the method begins with a leadframe strip formedof copper or another conductive material such as one of various metalsor metal alloys (1501). The strip is partially etched to form recessesor patterns defining the area of die attach pad 110 and electricalcontacts 125 (1502). The patterns can be formed using any of severalconventional etching techniques such as various forms of chemical ormechanical etching. Although not shown, the patterns can be definedinitially by forming an etch mask over the leadframe strip andperforming the etching based on the etch mask. Once the areas of dieattach pad 110 and electrical contacts 125 are formed, top and/or bottomsurfaces of those areas are plated with plating layer 135 using adouble-sided selective plating process (1503). Plating layer 135 canalso form a ground ring 245 on die attach pad for forming ground bondsfor IC chip 105. The plating layers 135 and 245 may comprise a metalsuch as a stack-up of nickel (Ni), palladium (Pd) and gold (Au), astack-up of nickel (Ni) and gold (Au) or silver (Ag). The plating layer135 on the bottom surface may be plated by the same metal layer as thetop surface, or with other metal finish such as silver (Ag), gold (Au),nickel (Ni) and gold (Au), or tin/lead (Sn/Pb) solder plating.

Referring to FIG. 15B, IC chip 105 is then attached, using an adhesivelayer 115, to the area defining die attach pad 110 (1504). The adhesivelayer 115 may comprise of either polymeric materials, such as epoxy,silicone, polyimide or thermoplastic materials (both in paste or filmform), or soft solder materials, such as gold-tin or variouscombinations of tin and lead alloys. Thereafter, wire bonds 130 areformed to connect IC chip 105 with electrical contacts 125 and groundring 245 (1505). Wire bonds 130 may be formed using a conventional wirebonding technology, such as, e.g., gold, copper or aluminum wirebonding.

Referring to FIG. 15C, molding layer 120 is then formed over wire bonds130, IC chip 105, and the areas defining die attach pad 110 andelectrical contacts 125. (1506). Thereafter, an etch-resist layer isselectively formed on the bottom surface of leadframe strip over theareas defining electrical contacts 125 (1507). The bottom surface of theleadframe strip is then back-etched to remove metal portions between theareas defining adjacent electrical contacts 125, between die attach pad110 and electrical contacts 125, and to partially remove a bottom metalportion of die attach pad 110 (1508). In the resulting structure, abottom surface of molding layer 120 is exposed through the removedportions between electrical contacts 125 and die attach pad 110 andbetween adjacent electrical contacts 125. Due to the back-etching, abottom surface of die attach pad 110 is substantially flush with thebottom surface of molding layer 120.

Referring to FIG. 15D, the etch-resist layer is stripped away fromelectrical contacts 125 to re-expose plating layer 135 on the bottomsurfaces thereof (1509). Once the etch-layer has been stripped away, theresulting package is severed from the rest of the leadframe strip by asingulation process such as saw singulation or mechanical punching(1510). The points of separation are indicated in FIG. 15D by a pair ofvertical bars. The separation results in a singulated IC package 200(1511). An enlarged version of the singulated unit is shown in FIG. 15Ewith the same labels as FIG. 2.

FIGS. 16A-16E illustrate another method of manufacturing a leadless ICpackage such as IC package 500 illustrated in FIG. 5. This method may beused to simultaneously manufacture several leadless IC packages with asingle leadframe strip.

Referring to FIG. 16A, the method begins with a leadframe strip formedof copper or another conductive material such as one of various metalsor metal alloys (1601). The strip is partially etched to form recessesor patterns defining the area of die attach pad 110 and electricalcontacts 125 (1602). The top and/or bottom surfaces of the areasdefining die attach pad 110 and electrical contacts 125 are then platedwith plating layer 135 using a double-sided selective plating process(1603). In the example of FIG. 16A, the same surfaces are plated as inthe example of FIG. 15A next to reference number 1503, except that inFIG. 16A, an additional portion is plated on the bottom of the areadefining die attach pad 110. Plating layers 135 and 245 may comprise ametal such as a stack-up of nickel (Ni), palladium (Pd) and gold (Au), astack-up of nickel (Ni) and gold (Au) or silver (Ag). Plating layer 135on the bottom surface may be plated by the same metal layer as the topsurface, or with other metal finish such as silver (Ag), gold (Au),nickel (Ni) and gold (Au), or tin/lead (Sn/Pb) solder plating.

Next, in FIG. 16B, IC chip 105 is attached, using an adhesive layer 115,to the area defining die attach pad 110 (1604). Adhesive layer 115 maycomprise polymeric materials such as epoxy, silicone, polyimide orthermoplastic materials (both in paste or film form), or soft soldermaterials such as gold-tin or various combinations of tin and leadalloys. Thereafter, wire bonds 130 are formed to connect IC chip 105with electrical contacts 125 and ground ring 245 (1605).

Subsequently, in FIG. 16C, molding layer 120 is formed over wire bonds130, IC chip 105, and the areas defining die attach pad 110 andelectrical contacts 125. (1606). An etch-resist layer is thenselectively formed on the bottom surface of leadframe strip over theareas defining electrical contacts 125 and a middle part of the areadefining die attach pad 110 (1607). The bottom surface of the leadframestrip is then back-etched to remove metal portions between the areasdefining die attach pad 110 and electrical contacts 125, and topartially remove a peripheral portion of die attach pad 110 (1608). Inthe resulting structure, a bottom surface of molding layer 120 isexposed through the removed portions between electrical contacts 125 anddie attach pad 110 and between adjacent contacts 125. In addition,peripheral portions 510 of die attach pad 110 are substantially flushwith the bottom surface of molding layer 120.

Referring to FIG. 16D, the etch-resist layer is stripped away fromelectrical contacts 125 and die attach pad 110 to re-expose platinglayer 135 on the bottom surfaces thereof (1609). The resulting packageis then severed from the rest of the leadframe strip by a singulationprocess such as saw singulation or mechanical punching (1610). Thepoints of separation are indicated in FIG. 16D by a pair of verticalbars. The separation results in a singulated IC package 500 (1611). Anenlarged version of the singulated IC package 500 is shown in FIG. 16Ewith the same labels as FIG. 5.

In view of the foregoing, it should be appreciated that leadless ICpackages can be formed with electrical contacts and die attach padshaving any of several different configurations to achieve a variety ofdifferent results. The different configurations can be achieved usingmanufacturing techniques such as those discussed above or any of severalvariations and/or substitutes available to those skilled in the art.

What is claimed:
 1. A leadless integrated circuit (IC) package,comprising: a die attach pad having a top surface and a bottom surface,the die attach pad being a single discrete element having a centerportion and a peripheral portion; an IC chip mounted on the top surfaceof the die attach pad in the peripheral portion thereof, the top surfaceof the die attach pad having an area larger than an area of the IC chipmounted thereagainst; a plurality of electrical contacts electricallyconnected to the IC chip; a molding layer formed over the IC chip, thedie attach pad, and the electrical contacts, the molding layer having atop part covering top portions of the electrical contacts and a bottompart through which the bottom surface of the die attach pad and bottomportions of the electrical contacts are exposed, the bottom part havingan exposed surface disposed between the die attach pad and electricalcontacts; a ground ring on the top surface of the peripheral portion ofthe die attach pad; wherein the bottom surface of the peripheral portionof the die attach pad is substantially flush with an exposed surface ofthe bottom part of the molding layer; wherein the bottom portions of oneor more of the plurality of electrical contacts extend out of the bottompart of the molding layer; and wherein the bottom surface of the centerportion of the die attach pad extends out of the bottom part of themolding layer by a distance less than a distance that the bottomportions of one or more of the electrical contacts extends out of theexposed surface of the bottom part of the molding layer.
 2. The ICpackage of claim 1, further comprising: a ground ring on the top surfaceof the peripheral portion of the die attach pad.
 3. The IC package ofclaim 2, wherein the ground ring is disposed on portion of the dieattach pad whose bottom surface is substantially flush with the bottompart of the molding layer.
 4. The IC package of claim 1, wherein theentire bottom surface of the center portion of the die attach pad issubstantially flat.
 5. The IC package of claim 1, wherein the bottomsurface of the center portion of the die attach pad has an area that issmaller than an area of the IC chip.
 6. The IC package of claim 1,wherein the bottom surface of the center portion of the die attach padcomprises a plurality of discrete protrusions.
 7. The IC package ofclaim 1, wherein the bottom surface of the center portion of the dieattach pad has an area that is smaller than the area of the IC chip. 8.The IC package of claim 7, wherein each of the electrical contacts has atop surface and a bottom surface, each of the bottom surfaces of theelectrical contacts having an area that is smaller than an area of thetop surfaces of the electrical contacts.
 9. The IC package of claim 8,further comprising a solder material formed on at least one of theelectrical contacts and the die attach pad.
 10. The IC package of claim9, wherein the solder material comprises a tin and lead solder alloy.